SOME EXAMPLES OF ROCK VARIATION 



381 



posed of bronzite and labradorite. These various mineralogical 

 types exhibit very interesting ranges in texture, to which atten- 

 tion will be called. The hornblende-gabbro of granitic texture 

 is the prevailing rock. An analysis of such a rock shows the 

 following composition : 



ANALYSIS OF HORNBLENDE-GABBRO BY MR. GEORGE STEIGER. 



Si0 2 

 Ti0 2 



A1 2 3 



Fe 2 ; 



FeO 



CaO 



MgO 



K,0 



2.22 



•13 



1. 71 



.07 



•15 



100.63 



In one of the hornblende gabbros a porphyritic texture is 

 very pronounced. Porphyritic brown hornblendes, which are 

 poikilitic from inclusions of feldspar and a few grains of augite, 

 lie in an imperfectly ophitic groundmass. This rock grades into 

 a finer grained nonporphyritic granitic textured gabbro. Both 

 mineralogical and textural variations are well shown in another 

 occurrence, in which the relations of the respective varieties 

 are clearly seen. The normal granitic hornblende-gabbro is cut 

 by numerous narrow bifurcating dikes, which are very dark, of 

 fine grain, and stand out clearly from the main mass of the 

 coarse-grained gabbro. These dikes contain a larger percentage 

 of biotite than is found in the normal gabbro, but most interest- 

 ing is the presence along the sides of the dikes of a well-marked 

 parallel arrangement of the minerals. This is presumed to be a 

 true flow structure consequent upon the flowage of the magma, 

 where it was forced in the fissures, as none of the minerals bear 

 indications of secondary origin, and show but faint evidence of 

 the effects of pressure. 



These dikes, as well as the main mass, are cut by a coarse- 

 grained bronzite-norite. Bronzite, hornblende, and labradorite 

 are the essential constituents of this rock, arranged in order of 



